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          <h1 class="post-title" itemprop="name headline">一次打包错误 引起的对 React Components, Elements, Instance三者的认识</h1>
        

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        <p>在一次项目重构中，我将filter组件需要的<code>Input</code>、<code>Select</code>之类通过<code>Connect</code>包裹后，通过<code>&lt;MyFromItem /&gt;</code>这种形式当作参数传入。</p>
<p>大致的数据结构如下，component可以传入自定义组件或者直接传入<code>&lt;Input /&gt;</code>这种形式；</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">[</span><br><span class="line">  &#123;</span><br><span class="line">    name: <span class="string">'name'</span>,</span><br><span class="line">    label: <span class="string">'姓名'</span>,</span><br><span class="line">    component: <span class="xml"><span class="tag">&lt;<span class="name">StaffFormItem</span> /&gt;</span></span>,</span><br><span class="line">  &#125;</span><br><span class="line">  ...</span><br><span class="line">]</span><br></pre></td></tr></table></figure>
<p>在本地跑的时候，并没有发现问题；然后打包之后就发现跑不通，报错<code>n is not a function</code>之类的错误；</p>
<p>打开sourcemap之后定位到这个n是rc-form中的getFieldDecorator函数，遂寻找这个值未传入的原因。</p>
<p>调试了多次（打包实在是太慢了……），终于发现是判断类型的时候发生了问题：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 这里传入了&lt;StaffFormItem /&gt;</span></span><br><span class="line"><span class="keyword">const</span> type = component.type.name;</span><br><span class="line">swith(type) &#123;</span><br><span class="line">  <span class="keyword">case</span> <span class="string">'Connect'</span>: <span class="comment">// 判断受控组件</span></span><br><span class="line">    <span class="keyword">return</span> <span class="comment">//...</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>打包之后，这里的type会从<code>Connect</code>变为<code>n</code>, <code>j</code>这种混淆后的形式，万万没想到。</p>
<p>解决问题之后去搜索了一下，出来了3个概念:</p>
<ul>
<li><p>Components</p>
</li>
<li><p>Component Instances</p>
</li>
<li><p>Elements</p>
</li>
</ul>
<p>在其他UI框架中的，一般只有模板（<code>Class</code>）和实例（<code>new Class()</code>）这样的概念；而在React中，<code>Component Instances</code>和<code>Elements</code>不是同一种类型的东西，它们之间没有一对一的关系；</p>
<p>看下面一段代码:</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> React, &#123; Component &#125; <span class="keyword">from</span> <span class="string">'react'</span>;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyComponent</span> <span class="keyword">extends</span> <span class="title">Component</span> </span>&#123;</span><br><span class="line">  <span class="keyword">constructor</span>(props) &#123;</span><br><span class="line">    <span class="keyword">super</span>(props);</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'这时候是Component Instance'</span>, <span class="keyword">this</span>);</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  render() &#123;</span><br><span class="line">    <span class="keyword">const</span> another_element = <span class="xml"><span class="tag">&lt;<span class="name">div</span>&gt;</span>Hello World!<span class="tag">&lt;/<span class="name">div</span>&gt;</span></span>;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'这里是一个element'</span>, another_element);</span><br><span class="line">    <span class="keyword">return</span> another_element;</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">'这是一个Component'</span>, MyComponent);</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> element = <span class="xml"><span class="tag">&lt;<span class="name">MyComponent</span> /&gt;</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">'这是一个element'</span>, element);</span><br></pre></td></tr></table></figure>
<p>上述这段代码，可以得出：</p>
<ul>
<li><p><code>MyComponent</code>(这个class自身)是一个Component</p>
</li>
<li><p><code>element</code>是一个对象，但是它不是MyComponent的一个实例。它仅仅描述了一个对象实例(<code>Component Instance</code>)被创建了，它是一个拥有<code>key</code>、<code>props</code>、<code>ref</code>、<code>type</code>这些属性的一个对象。在这里，<code>key</code>和<code>ref</code>的值为<code>null</code>,<code>props</code>是一个空对象，<code>type</code>即为<code>MyComponent</code>字符串。</p>
</li>
<li><p>一个<code>MyComponent</code>的实例将会在<code>element</code>被render之后被创建(在上述代码中，实例在构造函数中打印)</p>
</li>
<li><p><code>anthor_element</code>也是一个对象，它的<code>key</code>、<code>props</code>、<code>ref</code>和上面所述的一样，但是它的<code>type</code>为<code>div</code></p>
</li>
</ul>
<p>具体可以看<a href="https://reactjs.org/blog/2015/12/18/react-components-elements-and-instances.html" target="_blank" rel="noopener">React Components, Elements, and Instances</a></p>
<p>总结一下，可以看出React官方对这几个概念的定义非常清楚：</p>
<ul>
<li><p><code>An element is a plain object describing a component instance or DOM node and its desired properties.</code></p>
</li>
<li><p><code>A ReactElement is a light, stateless, immutable, virtual representation of a DOM Element</code></p>
</li>
</ul>
<p><code>Component</code>可以被用来创建一个<code>Instance</code>，当<code>Instance</code>被render之后就创建了一个<code>Element</code>。</p>
<p>创建实例的过程不需要我们手动进行；<br>多个Element可以描述同一个实例（比如<code>&lt;Parent /&gt;</code>组件的<code>render()</code>返回了<code>&lt;Child /&gt;</code>，每次触发<code>render</code>时都会返回一个新的<code>element</code>，但是已经存在<code>Child</code>实例可能被复用）；<br>一个Elmenet也可以用于描述多个实例（比如把一个<code>element</code>存储到一个变量中<code>const a = &lt;Child /&gt;</code>,然后调用多次<code>&lt;div&gt;{a} {a} {a}&lt;/div&gt;</code>）</p>

      
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